Vinyl ether group-containing (meth)acrylic esters have different kinds of polymerizable groups, namely radical- and anion-polymerizable (meth)acryloyl groups and cation-polymerizable vinyl ether groups, within molecules and, therefore, they are useful compounds which can be used in a wide range of industrial use as raw materials in medicinal chemicals and agrochemicals, as synthetic intermediates and further as polymerizable materials.
A number of studies have been made on the radical polymerizability and cation polymerizability of vinyl ether group-containing (meth)acrylic esters. However, because of their easy radical polymerizability and cation polymerizability, vinyl ether group-containing (meth)acrylic esters are poor in stability, which leads to impurity formation, peroxide formation and polymerization during storage or handling thereof, hence they can hardly be said to be put into fully practical use.
For methods of producing vinyl ether group-containing (meth)acrylic esters, there are known the method comprising subjecting (meth)acrylic acids and hydroxyl group-containing vinyl ethers to esterification (method A), the method comprising subjecting (meth)acrylic halides and hydroxyl group-containing vinyl ethers to esterification (method B), the method comprising subjecting (meth)acrylic anhydrides and hydroxyl group-containing vinyl ethers to esterification (method C), and the method comprising subjecting (meth)acrylic esters and hydroxyl group-containing vinyl ethers to transesterification (method D), and the like. The above esters can also be produced by the method comprising subjecting (meth)acrylic acids and halogen-containing vinyl ethers to esterification (method E) and by the method comprising subjecting (meth)acrylic acid alkali (or alkaline earth) metal salts and halogen-containing vinyl ethers to esterification (method F).
Of these production methods, those methods using halogen-containing vinyl ethers as starting materials cause the formation of an equimolar amount of by product salts. Therefore, those production methods using hydroxyl group-containing vinyl ethers as starting materials are suitable from the industrial viewpoint. Since, however, hydroxyl group-containing vinyl ethers are generally produced by the addition reaction of diol to acetylene (the so-called Reppe reaction) or by the gaseous phase dehydration reaction of diol-monoalkylene oxide adduct, a number of impurities are formed as byproducts. Since these impurities have properties such that are close in boiling point to hydroxyl group-containing vinyl ethers, form azeotropic compositions therewith and are close in polarity thereto, for instance, a complicated purification is required for the complete isolation to obtain pure hydroxyl group-containing vinyl ethers and cost of production of pure hydroxyl group-containing vinyl ethers is increased thereby. Consequently, the vinyl ether group-containing (meth)acrylic esters produced by using such hydroxyl group-containing vinyl ethers as starting materials become expensive. Thus, the advent of methods of producing vinyl ether group-containing (meth)acrylic esters at low cost using hydroxyl group-containing vinyl ethers as starting materials is awaited.
Another problem is that since vinyl ether group-containing (meth)acrylic esters, when produced by any of the methods (A) to (F), decompose during the process of production thereof, leading to impurity formation, peroxide formation and polymerization, it is impossible to stably produce them.
The method of producing vinyl ether group-containing (meth)acrylic esters which comprises subjecting (meth)acrylic ester and a hydroxyl group-containing vinyl ethers to transesterification is advantageous from the industrial viewpoint since it does not use any expensive or hazardous raw materials. However, it has the problem that the yields of vinyl ether group-containing (meth)acrylic esters is decreased by polymerization occurring in the reaction system. Further, there is another problem that peroxide formation due to the excess of molecular oxygen and impurity formation due to decomposition. There is a further problem that since the starting material hydroxyl group-containing vinyl ether, and the product vinyl ether group-containing (meth)acrylic ester each has a vinyl ether group, side reactions such as polymerization of the vinyl ether groups of the products and starting materials as caused by (meth)acrylic acid formed in trace amounts due to water occurring in the system and isomerization of the starting material hydroxyl group-containing vinyl ether to the corresponding 2-substituted-1,3-dioxo compound occur.
For the method of purifying the vinyl ether group-containing (meth)acrylic esters produced by such methods as mentioned above, there are known the techniques of extraction, washing with water, evaporation, distillation and column chromatography, etc. However, because of the easy radical polymerizability and cation polymerizability of vinyl ether group-containing (meth)acrylic esters, polymerization occurs in the process of purification and, further, impurity formation may easily occur as a result of decomposition. Accordingly, vinyl ether group-containing (meth)acrylic esters can hardly be said to be producible stably from the industrial viewpoint. The quality and storage stability problems of the product obtained still remain unsolved. Thus, there is room for investigation with a view to prevent the formation of impurities as a result of polymerization and decomposition of vinyl ether group-containing (meth)acrylic esters in the stages of storage, handling, production and purification thereof and thus render such vinyl ether group-containing (meth)acrylic esters fully practical for industrial use.
The present invention has been made in view of the above-mentioned state of the art. Accordingly, it is an object of the present invention to provide vinyl ether group-containing (meth)acrylic esters, which has both radical polymerizability and cation polymerizability, improved in storage stability and stability in handling without impairing its polymerizability or, in other words, provide stabilized vinyl ether group-containing (meth)acrylic esters. Another object is to provide methods of producing stabilized vinyl ether group-containing (meth)acrylic ester compositions. A further object is to provide methods of stably handing, methods of economically and stably producing and methods of purifying such vinyl ether group-containing (meth)acrylic esters.